A SAW device is constituted so that a pattern of interdigital transducer (hereinafter referred to as “IDT”) electrodes, a reflector, connection pads, and the like is arranged on a piezoelectric substrate consisting of quartz crystal, lithium tantalate, or the like. A SAW is activated by, for example, providing a high-frequency electric field to the IDT electrodes, and filter characteristics are obtained by converting the SAW into a high-frequency electric field by a piezoelectric effect.
Meanwhile, as a small-sized packaging technique referred to as “chip size package (CSP)” has been widely used for semiconductor components, a manufacturing method using the CSP technique has been introduced with a view of facilitating miniaturization of the SAW device, and improving productivity based on a batch manufacturing method.
CSP related techniques for the SAW device are disclosed in Japanese Patent Laid-Open Nos. 2001-176995 and 2002-184884, and International Patent Application publication No. WO97/02596.
FIG. 10 is a cross-sectional view which shows a SAW device disclosed in the Japanese Laid-Open No. 2001-176995. SAW chips 110 are provided on an each one piece domain of a sheet-shaped substrate 100 by flip-chip bonding. A film 120 which can be transformed by heat is provided over the SAW chips 110, and heated up to a softening temperature of the film, thereby the transformed film 120 is provided region between the SAW chips.
The sheet-shaped substrate 100 includes surface mounting external electrodes 102 provided on lower surfaces of respective insulating substrates 101 that constitute the respective piece domain, lands 103 conductive to the external electrodes 102 and provided on upper surfaces of the respective insulating substrates, and through holes 104 provided in boundaries among the respective insulating substrates 101, and letting the air out. Each SAW chip 110 includes connection pads 112 and an IDT electrode 113 on a lower surface of a piezoelectric substrate 111. By connecting the connection pads 112 to the respective lands 103 through conductor bumps 115, the SAW chip 110 is mounted on each insulating substrate 101. Further, by closely attaching the film 120 onto outer surfaces of all the SAW chips 110 and an upper surface of the mounting substrate through the above-stated steps of procedure, and then cutting and dividing the mounting substrate along boundary lines of the respective insulating substrates, pieces of SAW devices can be obtained.
According to this conventional technique, the through holes 104 are arranged in mounting substrate portions corresponding to valleys among the respective SAW chips 110. By performing negative pressure suction from the through holes 104 simultaneously with softening of the film 120, the film 120 can be closely attached onto outer surfaces of the SAW chips and the upper surface of the mounting substrate 101. As a result, an airtight space S for SAW propagation can be formed between the IDT electrode 113 and the upper surface of the insulating substrate 101.
According to this conventional technique, however, the air suction is performed not near the airtight space S but at the positions corresponding to the valleys among the SAW chips. Due to this, the film 120 cannot closely attached onto upper surfaces of the insulating substrates 101 since the film 120 cannot sufficiently be provided at end portion of SAW chips indicated by symbol A in the drawing. If an entry amount of the film 120 at the region A is insufficient, the film 120 cannot adhere to the insulating substrates 101. Then portions of the film 120 floating from the upper surfaces of the insulating substrates 101 are cut off when the mounting substrate is diced along the boundary lines of the insulating substrates 101 to be divided into the respective piece regions. As a result, holes communicating with the airtight space S may possibly be formed, and it is impossible to hold airtightness of space S. As a result, dustproofness and dampproofness of the SAW devices are disadvantageously and greatly deteriorated.
To cope with such disadvantages, the film of circumference of a SAW device is pushed to the insulating substrates 101 by means of a special die to obtain a rigid airtight structure. However, the die has a special shape, thereby disadvantageously causing cost increase. The cost increase is conspicuous particularly whenever a cutting pitch at which the mounting substrate 100 is cut into the respective piece regions is changed or whenever the size of the SAW chips is changed since different shape of dies are required.
The Japanese Patent Laid-Open No. 2002-184884 discloses a technique for enabling the resin heated and softened to sufficiently spread into the gaps A by providing the through holes 104 for letting air out of space S being arranged on a substrate 101 located underneath of a SAW chip. To do so, however, the through holes 104 need to be closed by plug members soon after the air is sucked from the through holes 104, which disadvantageously increases the number of steps. If a low viscosity resin is used as a material for the plug members, then the resin enters into the airtight space S and adheres to the IDT electrodes 113. As a result, SAW propagation may possibly, disadvantageously be disturbed. On the contrary, if a high viscosity resin is used as a material for the plug members, then moisture enters into the airtight space S from interfaces between the plug members and inner walls of the through holes 104 due to a low adhesiveness of the resin to the inner walls of the through holes. As a result, the disadvantage may possibly occur.
Further, the Japanese Patent Laid-Open No. 2002-184884 discloses, similarly to the Japanese Patent Laid-Open No. 2001-176995, a techniques for providing air extraction through holes in mounting substrate portions corresponding to the valleys among the SAW chips, sucking the air when the resin film is heated, and setting a temperature for deforming the resin film to be lower than a temperature at which the resin film is hardened. This conventional technique has the same disadvantage as that of the Japanese Patent Laid-Open No. 2001-176995 in that the resin cannot sufficiently enter into the gaps between the SAW chips. Besides, this conventional technique has the following disadvantage. The air in the airtight space S is thermally expanded when the temperature of the resin film is raised from the softening temperature of the resin to the hardening temperature of the resin. As a result, the resin provided between the gaps comes off from the top surface of the substrate 100, and voids tend to be generated. When so many voids are generated, holes are formed in the resin portions after the mounting substrate is diced into the respective piece regions, resulting in considerable deterioration of the dustproofness and the dampproofness of the SAW devices. In order to solve these disadvantages, it is necessary to reduce the pressure of the internal space so that the resin does not come off from the top surface of the substrate 100 even when the air within the airtight space is thermally expanded. This, in turn, makes it necessary to strictly manage temperature and suction profiles, resulting in complicated manufacturing steps and complicated manufacturing devices.
According to a manufacturing method disclosed in the International publication No. WO97/02596, while a thin sheet resin is provided on the upper surfaces of the SAW chips mounted on the insulating substrates by face-down mounting method and flip-chip bonding, the resin is heated up to the softening (melting) temperature, thereby closely attaching the sheet resin onto the top and side surfaces of the SAW chips and the upper surfaces of the insulating substrates with high viscosity kept. The temperature is further raised up to the hardening temperature, thereby fixing the shape of the resin. According to the manufacturing method of the International publication No. WO97/02596, when the resin is heated up to the softening temperature, the viscosity of the resin that has been softened is increased so as not to flow the resin into the gaps between the under surface of the SAW chips and the upper surfaces of the insulating substrates. Due to this, wettability of the softened resin to the insulating substrates is deteriorated, adhesiveness of the resin to the insulating substrates is deteriorated, and the resin is disadvantageously, easily separated from the insulating substrates. If the separation occurs even slightly, moisture tends to enter into the airtight space from interfaces between the resin and the insulating substrates. In order to solve this disadvantage, if the viscosity of the resin is reduced during the softening of the resin, the resin enters into the airtight space and tends to adhere to the IDT electrodes. This disadvantageously causes disturbance of the SAW propagation, and considerable deterioration of electric characteristics of the SAW devices.
The present invention has been achieved in view of the conventional disadvantages. It is an object of the present invention to provide a method of manufacturing a surface mount SAW device constituted so that SAW chips are mounted on a pattern of a mounting substrate through conductor bumps by face-down method and flip-chip bonding, a sheet resin provided on upper surfaces of the SAW chips is heated and softened to cover outer surfaces of the SAW chips with the sheet resin, and so that the resin is filled into gaps between the SAW chips and on an upper surface of the mounting substrate, thereby form an airtight space between IDT electrodes on lower surfaces of the SAW chips and the upper surface of the mounting substrate, wherein the method can dispense with negative pressure suction from through holes formed in the mounting substrate so as to ensure a filling amount of the resin into the gaps, and can dispense with strict management of heating temperature and suction profiles so as to ensure adhesiveness between the resin and the mounting substrate.